Multifunctional Janus composite diaphragm, preparation method and application

Abstract

The invention discloses a multifunctional Janus composite diaphragm, a preparation method and application, the diaphragm is formed by coating two sides of a Celgard diaphragm with different functional layers, one side is a Fe / Bi-S-C coating layer, the Fe / Bi-S-C coating layer obtains hydrogel microspheres through a sol-gel reaction of chitosan, dopamine and a metal salt precursor, the hydrogel microspheres are subjected to freeze drying, carbonization and sulfuration treatment, and the multifunctional Janus composite diaphragm is obtained. FeS and Bi2S3 nanoparticles are generated in situ in the carbon skeleton; and a stable Van der Waals heterojunction layer assembled by BN and MXene nanosheets is arranged on the other side of the Van der Waals heterojunction layer. According to the Janus composite diaphragm disclosed by the invention, through the synergistic strengthening effect of the functional layers coated on the two sides, the diaphragm not only can effectively inhibit the generation and growth of lithium dendrites, but also can efficiently capture and catalyze the conversion of polysulfides, so that the cycling stability and sulfur utilization rate of a battery are remarkably improved. The diaphragm realizes collaborative optimization in the aspects of mechanical stability, thermal safety and electrochemical performance.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and particularly relates to a multifunctional Janus composite separator, its preparation method, and its application. Background Technology

[0002] As a key component separating the positive and negative electrodes and enabling lithium-ion transport, the separator has a significant impact on battery safety. Although traditional polyethylene (PE) or polypropylene (PP) separators have certain mechanical strength and thermal shutdown capabilities, they suffer from low porosity, poor wettability, and insufficient thermal stability. They are prone to shrinkage or melting at high temperatures, leading to increased electrode contact and safety risks.

[0003] To meet the demands of high-safety, high-performance battery systems, researchers have proposed the Janus composite separator structure. This structure employs asymmetric design to impart different physical or chemical functions to both sides of the separator, balancing multiple requirements such as ion conduction, thermal stability, and interfacial compatibility. Existing Janus composite separators typically utilize coating, spinning, or composite processes, introducing a heat-resistant layer (such as ceramic or aramid fibers) on one side to enhance thermal stability, while the other side is designed as an interfacial layer with high wettability or specific ion-sieving functions. However, current Janus separators used in lithium-ion batteries mostly employ carbon-based, MXene, or metal oxide composite structures. While these can improve thermal stability and suppress dendrite growth to some extent, they still have several shortcomings: their functions are usually relatively singular, interfacial bonding is weak, and delamination or peeling easily occurs during long-term cycling; dense coatings restrict ion transport and hinder electrolyte wetting; and insufficient thermal conductivity and low thermal diffusion efficiency make it difficult for the separator to effectively cope with the risk of thermal runaway under high energy density conditions. More importantly, the Janus separator used in traditional lithium-ion batteries lacks targeted chemical adsorption and catalytic functions on the positive electrode side, which cannot suppress the migration of active materials or the generation of side reactions, thus limiting its electrochemical performance and cycle stability.

[0004] Especially in lithium-sulfur batteries, in addition to the conventional risk of thermal runaway, there are also problems such as the flammability of elemental sulfur, the high reactivity of lithium metal anodes, the low flash point of ether electrolytes, and the strong oxidizing properties of LiNO3 additives. These issues place higher demands on the thermal stability, flame retardancy, and polysulfide blocking capabilities of the separator. Therefore, there is an urgent need to develop a multifunctional Janus composite separator that combines high ion conductivity, excellent thermal stability, flame retardancy, and polysulfide suppression capabilities to simultaneously improve the safety performance and electrochemical stability of both lithium-ion and lithium-sulfur batteries. Summary of the Invention

[0005] The main objective of this invention is to provide a multifunctional Janus composite separator that combines high ion conductivity, excellent thermal stability, flame retardant properties, and polysulfide suppression capabilities, thereby simultaneously improving the safety performance and electrochemical stability of lithium-ion and lithium-sulfur batteries.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a multifunctional Janus composite membrane includes the following steps: Step S1: Prepare hydrogel microspheres; the raw materials used are chitosan, dopamine, ferric nitrate nonahydrate and bismuth nitrate pentahydrate; Step S2, carbonization and sulfidation treatment; the hydrogel microspheres prepared in step S1 are calcined at high temperature in a nitrogen atmosphere to obtain carbonized aerogel; after calcination, high temperature sulfidation is carried out in a sulfur vapor atmosphere to obtain metal sulfide / carbon composite aerogel with metal sulfide in situ composite on carbon skeleton, denoted as Fe / Bi-S@C; the composite aerogel is centrifuged, washed with anhydrous ethanol and dried. Step S3: Prepare boron nitride nanosheets and MXene nanosheets; Step S4: Mix the MXene nanosheets and boron nitride nanosheets prepared in step S3 with a bio-based antioxidant (such as plant polyphenols, catechins or green tea extract) and place them in a vacuum ball mill jar for ball milling; after ball milling, heat and keep warm in an inert atmosphere to obtain BN–MXene heterojunction powder. This approach introduces bio-based antioxidants (such as plant polyphenols, catechins, and green tea extracts) into the preparation process of heterojunctions, and achieves comprehensive optimization of heterojunction performance through interface modification, structural regulation, and multifunctional synergistic protection.

[0007] Firstly, regarding interface modification, bio-based antioxidants not only effectively inhibit the surface oxidation of MXene but also act as interfacial binders, forming hydrogen bonds, π–π interactions, and weak van der Waals interactions between MXene and h-BN to construct a stable MXene-antioxidant-h-BN "sandwich" interface structure. This structure prevents MXene layer oxidation and delamination while significantly enhancing the overall bonding strength and structural stability of the heterojunction. Secondly, regarding ball milling structure modification, antioxidants can form a uniform thin film or local adsorption layer on the particle surface during ball milling, regulating the dispersion and distribution density of MXene and h-BN, preventing MXene from being directly exposed to air, thereby effectively improving the oxygen barrier capacity and interfacial uniformity of the system. Finally, in terms of multifunctional synergistic protection, antioxidants and h-BN work synergistically: on the one hand, the layered structure of h-BN provides a physical barrier to prevent oxygen and moisture from penetrating; on the other hand, antioxidants capture oxygen free radicals and water molecules through chemical passivation reactions, further preventing MXene oxidation; at the same time, active groups such as hydroxyl groups and benzene rings in its molecular structure can enhance the interfacial binding force between MXene and h-BN, achieving a triple protection effect of physical isolation, chemical stability and interfacial stability.

[0008] Step S5: Dissolve the composite aerogel prepared in step S2 and polyvinylidene fluoride in N-methylpyrrolidone solution, stir at room temperature to obtain a uniform dispersion; coat the dispersion onto one side of a commercial membrane by vacuum filtration, deposit metal sulfide / carbon composite aerogel particles on this side of the commercial membrane, and dry it. Step S6: Dissolve the BN–MXene heterojunction powder prepared in step S4 and polyvinylidene fluoride in an N-methylpyrrolidone solution, stir at room temperature to obtain a uniform dispersion; coat the dispersion onto the other side of a commercial membrane by vacuum filtration, deposit the BN–MXene heterojunction on this side of the commercial membrane, and dry it.

[0009] Furthermore, in the Janus composite membrane obtained after step 6, the loading amount of the Fe / Bi-S@C and BN–MXene heterojunctions on both sides of the membrane is 1.5 mg cm⁻¹ on each side. -2 .

[0010] Specifically, step S1 is performed as follows: 1.5 g chitosan, 0.5 g dopamine, 0.5 g ferric nitrate nonahydrate, and 0.5 g bismuth nitrate pentahydrate are dissolved in 50 mL of 5.0% acetic acid solution, and the solution is magnetically stirred at 180 rpm for 24 h to obtain a homogeneous solution; subsequently, the solution is diluted with 3.5 mL of water per minute. -1The solution was slowly added dropwise to a 10% sodium hydroxide solution and aged for 24 h. The resulting hydrogel microspheres were washed with deionized water until the filtrate was neutral. Then, the hydrogel microspheres were placed in 100 mL of a 2% glutaraldehyde solution and stirred for 2 h. The product was dried, washed with anhydrous ethanol, and then freeze-dried for 12 h.

[0011] All reactants (chitosan, dopamine, iron source, and bismuth source) are thoroughly mixed in an acetic acid solution to ensure uniform distribution at the molecular / ionic level. In an acidic environment, the amino groups of chitosan and the catechol and amino functional groups of dopamine begin to coordinate with iron and bismuth ions, forming preliminary metal-organic complexes. In an alkaline environment, especially in the presence of Fe³⁺, dopamine is rapidly oxidized to dopaquinone, which then undergoes a complex polymerization reaction to generate polydopamine. The generated polydopamine can adhere tightly to the chitosan gel backbone or covalently / non-covalently bind to it, further enhancing the gel's strength and adhesion. Simultaneously, polydopamine itself contains numerous functional groups that can complex metal ions.

[0012] In a strongly alkaline environment, metal ions in the solution react with OH⁻ to form insoluble hydroxide precipitates. These metal hydroxide nanoparticles or clusters are then encapsulated in situ within the forming chitosan / polydopamine gel network. This uniform encapsulation prevents the hydroxide particles from agglomerating, ensuring their nanoscale dispersion within the composite material.

[0013] The cross-linking effect of glutaraldehyde as a cross-linking agent on hydrogel microspheres can achieve the following effects: Enhanced mechanical strength: The cross-linked hydrogel network becomes more robust, able to withstand greater external forces without easily breaking, and the integrity of the microspheres is greatly improved.

[0014] Improved structural stability: The cross-linked hydrogel is less prone to swelling, deformation, or dissolution in water or electrolyte during subsequent drying, washing, and final application. This ensures the long-term stability of its microstructure and function.

[0015] Controlling pore structure: The degree of cross-linking can regulate the compactness of the hydrogel network, thereby affecting its final pore size and distribution.

[0016] Specifically, the carbonization process in step S2 is as follows: the hydrogel microspheres prepared in step S1 are placed in a tube furnace and heated at 3 °C for 3 min under a nitrogen atmosphere. -1The temperature was increased to 800 °C and held for 2 h. Then, under continuous nitrogen atmosphere protection, the tube furnace was allowed to cool naturally to room temperature to obtain carbonized aerogel. The specific procedure for vulcanization is as follows: the carbonized aerogel is placed in a tube furnace containing sulfur powder, and nitrogen is continuously introduced. First, the low-temperature zone where the sulfur powder is located is heated to generate sulfur vapor. Then, the high-temperature zone where the carbonized aerogel is located is heated to 600 °C and held for 2 h.

[0017] The purpose of carbonization is to transform the organic hydrogel precursor into a conductive, porous inorganic carbon framework and to reduce the metal species. Its functions and effects are: 1. Formation of a porous carbon framework: When hydrogel microspheres are heated to a high temperature of 800°C under an inert atmosphere (N2), the organic components within them undergo pyrolysis and carbonization.

[0018] Volatile substances (such as H2O, CO, CO2, and small molecule hydrocarbons) are released, leaving behind a carbon-rich carbon aerogel with a high specific surface area and porous structure. The porous structure formed during the freeze-drying process is largely preserved or further evolved.

[0019] 2. Reduction of metal oxides: In the preceding steps, the metal ions have been converted into hydroxides / oxides. During the carbonization process at 800°C, these metal oxides undergo a carbothermic reduction reaction with the surrounding pyrolyzed carbon.

[0020] Effect: Zero-valent metal (Sn, Fe) nanoparticles are generated and embedded in a carbon matrix to obtain metal / carbon composite aerogels.

[0021] 3. Enhances electrical conductivity and mechanical stability: High-temperature carbonization graphitizes amorphous carbon, significantly improving the electronic conductivity of the material.

[0022] The carbon skeleton provides mechanical support for the entire structure, preventing it from collapsing during subsequent handling and use.

[0023] The purpose of sulfidation is to convert metal nanoparticles in a carbon matrix into target metal sulfides, which is key to obtaining specific electrochemical / catalytic properties. Its functions and effects are: 1. In-situ sulfidation reaction: When sulfidation is carried out at 600°C (usually in an atmosphere containing sulfur vapor, such as by sublimation of sulfur powder), sulfur diffuses into the interior of the carbon matrix and reacts with the previously generated metal (Bi,Fe) nanoparticles.

[0024] Effect: Converts metal particles into metal sulfide nanoparticles.

[0025] 2. Formation of a unique "confined" structure: This is an in-situ transformation process. The resulting sulfide nanoparticles remain confined within the pores of the carbon matrix, a structure with significant advantages: Preventing aggregation: The carbon matrix acts as a physical barrier, preventing the high surface energy sulfide nanoparticles from migrating and agglomerating at high temperatures, thus maintaining a small particle size and a high active area.

[0026] Buffering volume changes: When used as battery electrode materials, sulfides undergo significant volume expansion / contraction during charge and discharge. The carbon skeleton can effectively buffer this stress, prevent electrode structure from shattering, and thus improve cycle stability.

[0027] Enhanced conductivity: The conductive carbon network provides a fast electron transport path for sulfides that have insulating or semiconductor properties.

[0028] After two steps of "carbonization + sulfidation", a metal sulfide / carbon composite aerogel is finally obtained. The characteristics and advantages of the final product are as follows: High specific surface area and hierarchical porous structure: derived from aerogel precursor, which is beneficial for electrolyte penetration and rapid ion transport.

[0029] Excellent conductive network: The continuous carbon skeleton ensures excellent electronic conductivity.

[0030] Uniformly dispersed active materials: Ultra-small metal sulfide nanoparticles are uniformly "confined" in a carbon matrix, with active sites fully exposed.

[0031] Strong structural stability: The carbon skeleton serves as both a conductive network and a mechanical support and buffer layer.

[0032] Specifically, the preparation process of boron nitride nanosheets in step S3 includes: Hexagonal boron nitride micro powder was placed in a quartz tube in the center of a tube furnace. The tube furnace was heated from room temperature to 800~1200℃ at a heating rate of 5℃ / min and held at that temperature for 2~4 h. After the tube furnace cooled naturally to room temperature, the quartz tube was removed to obtain the hydroxylated boron nitride product, denoted as BNO. BNO powder was added to a toluene solution, stirred and mixed, and the temperature was raised to 80-100 °C. KH-560 was then added, and the mixture was refluxed for 10-15 h. After the reaction was completed, the reaction solution was cooled to room temperature and then vacuum filtered. The precipitate remaining after filtration was collected and repeatedly washed with ethanol to remove residual toluene solvent and unreacted KH-560, finally obtaining modified boron nitride nanosheets.

[0033] This method first hydroxylates h-BN powder, introducing hydroxyl (-OH) functional groups to fundamentally alter the surface chemical properties of boron nitride, thereby significantly improving its dispersibility in solvents, compatibility with other materials, and chemical reactivity. BNO powder is initially dispersed in toluene solution. While the original BN is hydrophobic, the hydroxylated BNO (hydroxylated boron nitride) becomes hydrophilic. In toluene, a non-polar solvent, BNO itself may not disperse easily; however, mechanical stirring allows for particle separation and suspension in the solvent, preventing the formation of hard precipitates and maximizing the specific surface area of ​​BNO, thus providing more opportunities for the hydroxyl groups on its surface to react with KH-560. Toluene effectively dissolves the subsequently added silane coupling agent KH-560.

[0034] Through a chemical reaction, KH-560 molecules are covalently grafted onto the surface of hydroxylated boron nitride, thereby transforming it from hydrophilic BNO into hydrophobic BN nanosheets with active epoxy groups, resulting in hydrophobic and oleophilic properties, which greatly improves its dispersibility and interfacial compatibility in organic systems.

[0035] A multifunctional Janus composite separator is disclosed, comprising a Celgard separator coated with different functional layers on both sides: one side is a Fe / Bi-S@C coating layer, in which hydrogel microspheres are obtained through a sol-gel reaction of chitosan, dopamine, and metal salt precursors, followed by freeze-drying, carbonization, and sulfidation to generate FeS and Bi2S3 nanoparticles in situ within the carbon framework; the other side is a heterostructure coating layer, which is a stable van der Waals heterojunction layer assembled from BN and MXene nanosheets. The synergistic effect of the two functional layers not only significantly improves the separator's porosity, liquid absorption rate, and ionic conductivity, but also endows it with excellent thermal stability, flame retardancy, and the ability to inhibit lithium dendrite growth, thereby comprehensively improving the electrochemical performance and safety of lithium-ion batteries.

[0036] A multifunctional Janus composite separator is used as a battery separator in lithium-ion batteries. The synergistic effect of the two functional layers endows the separator with excellent porosity, liquid absorption, and ionic conductivity, while also giving it outstanding thermal stability, flame retardancy, and the ability to inhibit lithium dendrite growth, thereby comprehensively improving the electrochemical performance and safety of lithium-sulfur batteries.

[0037] The technical principle is as follows: The frog-egg-like carbon structure, with its abundant porosity and hollow features, can effectively accommodate sulfur and its discharge products, mitigating the volume expansion of sulfides during charging and discharging. Simultaneously, its three-dimensionally interconnected porous channels facilitate electrolyte wetting and rapid ion transport, enhancing the overall reaction kinetics of the electrode. The nitrogen and sulfur heteroatoms introduced into the carbon framework provide abundant polar active sites, forming N–Li and S–Li bonds and chemisorption with lithium polysulfides (LiPSs), thereby inhibiting the dissolution and diffusion of LiPSs in the electrolyte and reducing capacity decay caused by the shuttle effect. Furthermore, doping with heteroatoms can regulate the electron cloud distribution of the carbon structure, enhancing polar interactions and further promoting LiPS anchoring and electron transport. Introducing bimetallic sulfides into the carbon-based structure provides abundant surface metal active centers that can serve as catalytic sites, accelerating the reversible Li₂S₄ / Li₂S reaction, improving the conversion efficiency of polysulfides and the actual utilization rate of sulfur. The synergistic effect of carbon-heteroatoms-bimetallic sulfides not only stabilizes the electrode interface but also significantly improves the rate performance and cycle life of lithium-sulfur batteries.

[0038] BN–MXene heterostructures play a crucial role in suppressing lithium dendrite formation and thermal management. BN possesses extremely high thermal conductivity (≈51.7 W·m). -1 ·K -1 With its excellent thermal stability (decomposition temperature up to 2800 ℃ in N2 atmosphere), it can rapidly dissipate local Joule heat, avoiding the accumulation of "hot spots" caused by overheating, thereby preventing the local accelerated growth of dendrites. Simultaneously, the BN layer, as a dense physical barrier, can homogenize the local current density and Li⁺ flux, reducing the probability of dendrite nucleation. MXene materials possess excellent electronic conductivity and lithiophilicity, effectively regulating lithium-ion deposition behavior and promoting uniform lithium deposition. The heterojunction structure formed at the BN–MXene interface combines high thermal conductivity and high electrical conductivity; its high Young's modulus (≈8.7 GPa) can mechanically resist dendrite penetration, improving the structural stability of the separator. The synergistic effect of these two factors achieves rapid electron and heat transfer, uniform lithium-ion distribution, and stable interface evolution, thereby significantly suppressing dendrite growth and improving battery safety and cycle stability.

[0039] The two types of coatings work synergistically with the substrate separator to suppress LiPSs shuttle on the positive electrode side through the "adsorption-catalysis" dual function of the frog egg-like structure, thereby improving the battery's cycle stability and active material utilization. On the negative electrode side, the triple advantages of the BN-MXene heterojunction in "thermal conductivity-mechanical barrier-ion regulation" suppress lithium dendrite growth and ensure uniform heat distribution. When the battery faces the risk of thermal runaway, the frog egg-like carbon matrix can promote the formation of a dense carbon layer, isolating oxygen and heat transfer, while the BN-MXene heterojunction, with its high thermal stability, delays the separator collapse and avoids direct contact and short circuit between the positive and negative electrodes. Together, they interrupt the thermal runaway chain reaction, significantly improving the electrochemical performance and safety performance of the lithium-sulfur battery.

[0040] The beneficial effects of this invention are: Compared to the limitations of traditional single-function separators in simultaneously addressing the shuttle effect, lithium dendrite formation, and thermal runaway, the Janus composite separator described in this invention, through the synergistic reinforcement of functional layers coated on both sides, not only effectively inhibits the formation and growth of lithium dendrites but also efficiently captures and catalyzes the conversion of polysulfides, thereby significantly improving the battery's cycle stability and sulfur utilization. First, the heterojunction layer structure endows the separator with excellent mechanical strength and electron / ion transport stability, resisting dendrite penetration and suppressing short-circuit risks during long-term cycling. Second, the high thermal conductivity of the heterojunction facilitates rapid and uniform heat diffusion and distribution within the battery, preventing the accumulation of local hot spots and significantly reducing the probability of thermal runaway. Third, the synergistic structure of the porous carbon framework and bimetallic sulfides provides abundant electrocatalytic active sites, enabling efficient adsorption and catalysis of lithium polysulfide conversion reactions, reducing capacity decay caused by the shuttle effect. Through these combined effects, the separator achieves synergistic optimization in mechanical stability, thermal safety, and electrochemical performance. Attached Figure Description

[0041] Figure 1 (a) is a SEM image of Fe / Bi-S@C prepared in Example 1 of this application, and (b) is an elemental mapping diagram of Fe / Bi-S@C; Figure 2 SEM images of the BN and BN-MXene heterojunctions prepared in Example 1 of this application are shown, where (a) and (c) are SEM images of BN at different scales, and (b) and (d) are SEM images of BN-MXene at different scales. Figure 3 SEM images of the Janus composite membrane prepared in Example 1 of this application are shown, where (a) and (b) are SEM images of the surface of the BN-Mxene membrane at different scales, (c) is a cross-sectional view of the Janus modified membrane, and (d) and (e) are SEM images of the surface of the Fe / Bi-S@C membrane at different scales. Figure 4Peel strength diagrams of the bonding forces at the Fe / Bi-S@C layer-Celgard interface and the BN / MXene layer-Celgard interface; Figure 5 Thermal conductivity diagrams for Celgard and Janus membranes; Figure 6 The diagram shows the ionic conductivity of two stainless steel symmetrical cells. Figure 7 The graph shows the results of cycling two types of lithium-sulfur batteries 100 times at a current density of 0.1C. Figure 8 The graph shows the results of cycling two types of lithium-sulfur batteries for 150 cycles at a current density of 0.1-0.5C. Figure 9 The cycling performance of two lithium-sulfur batteries after 500 cycles at a current density of 0.1-1C is shown in the figure. Figure 10 The rate performance of two lithium-sulfur batteries after 31 cycles at current densities of 0.1–4.0–0.1C is shown in the figure. Figure 11 The GCD curves of two lithium-sulfur batteries at a current density of 0.1C are shown, where (a) is the Celgard battery and (b) is the Janus battery. Figure 12 The Q values ​​for two lithium-sulfur batteries at a current density of 0.1C are given. L and Q H Value, where (a) is the discharge capacity Q corresponding to the high voltage platform. H (b) represents the discharge capacity Q corresponding to the low-voltage platform. L ; Figure 13 The Q values ​​for two lithium-sulfur batteries at a current density of 0.1C are given. L / Q H and Q L / 3Q H Value, where (a) is Q L / Q H Value, (b) is Q L / 3Q H value; Figure 14 (a) shows the Li+ stripping / electroplation performance of the two lithium copper batteries, and (b) and (c) show the voltage changes of the two lithium copper batteries at 96-116 h and 208-224 h. Figure 15 (a) shows the coulombic efficiency of two lithium symmetric batteries, (b) shows the voltage-time curve of the Janus lithium symmetric battery, and (c) shows the voltage-time curve of the Celgard lithium symmetric battery. Figure 16(a) shows the adiabatic thermal runaway temperature curves of the Celgard lithium-sulfur battery after freshness and cycling, and (b) shows the adiabatic thermal runaway temperature curves of the Janus lithium-sulfur battery after freshness and cycling. Figure 17 (a) shows the adiabatic thermal runaway heat release rate curves of Celgard lithium-sulfur batteries after freshness and cycling, and (b) shows the adiabatic thermal runaway heat release rate curves of Janus lithium-sulfur batteries after freshness and cycling. Figure 18 (a) is a graph showing ln(dT / dt) vs 1000 / T for four lithium-sulfur batteries, and (b) is a bar chart showing the activation energy and total heat release for four lithium-sulfur batteries. Detailed Implementation

[0042] To further illustrate the technical solution of the present invention, preferred embodiments are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0043] Furthermore, all raw materials or reagents mentioned below that are not specifically described are commercially available products, and all process steps or methods not specifically mentioned are process steps or methods known to those skilled in the art.

[0044] The sources of some of the raw materials and reagents used in the following examples, comparative examples, and test cases are shown below: Chitosan was purchased from Aladdin Reagent Co., Ltd., model AR, 98wt.

[0045] Dopamine was purchased from Sinopharm Chemical Reagent Co., Ltd., model AR, 99wt.

[0046] Ferric nitrate nonahydrate was purchased from Aladdin Biochemical Technology Co., Ltd., model AR, 98wt.

[0047] Bismuth nitrate pentahydrate was purchased from Aladdin Biochemical Technology Co., Ltd., model AR, 99.9 wt.

[0048] Acetic acid solution was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., model A291609, with a purity of 5.00% (w / w).

[0049] Sodium hydroxide solution was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., model S817976-1L, concentration 0.1mol / L.

[0050] Glutaraldehyde solution was purchased from Shanghai Xinyu Biotechnology Co., Ltd., model AR, 99wt.

[0051] Hexagonal boron nitride powder (h-BN) was purchased from China National Pharmaceutical Chemical Reagent Co., Ltd., model AR, 99wt.

[0052] Formaldehyde was purchased from China National Pharmaceutical Chemical Reagent Co., Ltd., with a concentration of 37.0-40.0% and model number AR.

[0053] γ-glycidoxypropyltrimethoxysilane (KH-560) was purchased from Sinopharm Chemical Reagent Co., Ltd., model AR, with a purity ≥98.0%.

[0054] Ti3AlC2 powder was purchased from Ningbo Beijiaer New Material Co., Ltd., model AR, 98wt.

[0055] The lithium fluoride was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., model AR, with a purity of ≥99.0%.

[0056] The polytetrafluoroethylene was purchased from Shanghai Aladdin Reagent Co., Ltd., model AR, with a purity ≥99.0%.

[0057] Hydrochloric acid was purchased from Sinopharm Chemical Reagent Co., Ltd., with a concentration of 36.0%~38.0%. The polyvinylidene fluoride was purchased from Zhejiang Funolin Chemical New Materials Co., Ltd., model FN-PVDF-H10, with a purity of 99.9%.

[0058] N-methylpyrrolidone was purchased from Hubei Tuoyuan Fine Chemical Co., Ltd., and was of industrial grade with a purity of ≥99.8%.

[0059] The plant polyphenols were purchased from Shanghai Aladdin Reagent Co., Ltd., model number AR, purity 99%.

[0060] Example 1: The method in this example includes the following steps: Step S1: Dissolve 1.5 g chitosan, 0.5 g dopamine, 0.5 g ferric nitrate nonahydrate, and 0.5 g bismuth nitrate pentahydrate in 50 mL of 5.0% (v / v) acetic acid solution and magnetically stir at 180 rpm for 24 h to obtain a homogeneous solution. Then, dilute the solution with 3.5 mL of water per minute. -1 The precursor was slowly added dropwise to a sodium hydroxide solution (10% w / v, 10 g NaOH in 100 mL of solution) to prepare hydrogel microspheres, which were then aged for 24 h. The precursor, i.e., the hydrogel microspheres, was washed with deionized water until the filtrate was neutral.

[0061] Then, the hydrogel microspheres were placed in 100 mL of glutaraldehyde solution (2% w / v) and stirred for 2 h to achieve complete cross-linking. They were then placed in an atmospheric pressure blower dryer at a temperature controlled at 40-60℃ with the blower speed set to medium-low for approximately 1-3 h. After drying, the microspheres were washed with anhydrous ethanol. Finally, they were freeze-dried for 12 h.

[0062] Step S2: Place the hydrogel microspheres into a tube furnace and incubate at 3 °C for 3 min under a N2 atmosphere. -1 The temperature was increased to 800 °C and held for 2 h for calcination to obtain carbonized aerogel. Under continuous N2 atmosphere protection, the tube furnace was allowed to cool naturally or the program was set to cool to room temperature.

[0063] Afterwards, once the tubular furnace has cooled to room temperature, it is reheated to 600 °C at a rate of 2 °C / min and held at 600 °C for 2 h for vulcanization.

[0064] Finally, after centrifugation, washing with anhydrous ethanol, and drying at 60 °C for 12 h, frog-egg-like Fe / Bi-S@C was obtained, such as... Figure 1 As shown in (a) and (b).

[0065] Specific procedures for vulcanization treatment: The carbonized aerogel obtained in the previous step is placed in one ceramic boat, and the excess sulfur powder is placed in another ceramic boat. Both ceramic boats are then placed together into the quartz tube of a tube furnace. The ceramic boat containing the carbonized aerogel is placed in the high-temperature zone of the tube furnace (i.e., the area where the temperature is precisely controlled to 600°C), while the ceramic boat containing the sulfur powder is placed in the upstream low-temperature zone of the tube furnace.

[0066] First, introduce nitrogen gas to purge the air; Then, the low-temperature zone containing the sulfur powder is first heated to 150-200℃. At this temperature, the sulfur powder will sublimate, producing a large amount of sulfur vapor.

[0067] A continuous flow of nitrogen gas acts as a carrier gas, carrying sulfur vapor to a downstream high-temperature zone where it reacts with carbonized aerogel at 600°C.

[0068] At a high temperature of 600°C, sulfur vapor reacts with metal components such as iron and bismuth in the carbonized aerogel to generate corresponding sulfides, which are then in situ composited onto the carbon skeleton, ultimately yielding the Fe / Bi-S@C composite material.

[0069] Step S3, preparation of boron nitride nanosheets: First, 10 g of h-BN powder (hexagonal boron nitride micro powder) was placed in the quartz tube in the center of a tube furnace. Under an air atmosphere, the tube furnace was heated from room temperature to 1000 ℃ at a heating rate of 5 ℃ / min and held at 1000 ℃ for 2 h. After the tube furnace cooled naturally to room temperature, the quartz tube was removed to obtain the hydroxylated boron nitride product, denoted as BNO. Subsequently, ionic liquid modification was performed. 10 g of the prepared BNO powder was added to a three-necked flask containing 200 mL of 99% toluene solution and stirred until homogeneous. Then, the temperature of the reaction system was raised to 90 ℃, and 2 mL of the solution was slowly added while stirring. KH-560 (γ-(2,3-epoxypropoxy)propyltrimethoxysilane) was reacted under reflux for 12 h. After the reaction, the reaction solution was cooled to room temperature and then vacuum filtered. The precipitate was collected and repeatedly washed with ethanol to remove residual toluene solvent and unreacted KH-560, finally yielding BN nanosheets, such as... Figure 2 As shown in (a) and (c).

[0070] Reflux refers to the operating state in which, while heating and boiling the reaction liquid, the evaporated vapor is cooled and reliquefied through a reflux condenser, allowing it to flow back into the reaction vessel, thus preventing the loss of reaction solvents or reagents due to evaporation.

[0071] Preparation of MXene nanosheets: 20 mL of 9 M HCl solution and 1.6 g of LiF were added to a 150 mL polytetrafluoroethylene beaker and magnetically stirred for 5 min until homogeneous. Then, 1 g of Ti3AlC2 powder was gradually added to the mixture, and the system temperature was controlled at 35 °C with continuous stirring for 24 h. After the reaction, the resulting mixture was centrifuged at 5000 rpm for 5 min, and the bottom precipitate was collected. The precipitate was repeatedly washed with deionized water until the supernatant was neutral. To promote product exfoliation, the neutral precipitate was redispersed in 100 mL of deionized water to form Ti3C2T x The slurry was ultrasonically treated for 1 h under cold water bath conditions. After ultrasonic treatment, the treated slurry was centrifuged at 3500 rpm for 5 min, and the supernatant was freeze-dried to finally obtain MXene nanosheets.

[0072] In step S4, 9 g of MXene nanosheets, 1 g of BN nanosheets, and 0.2 g of plant polyphenols were mixed and placed in a vacuum ball mill jar, and ball milled at 500 rpm for 12 h. After ball milling, the resulting product was subjected to inert atmosphere at 2 °C for 1 min. -1The temperature was increased to 300 °C and held for 3 h to obtain BN–MXene heterojunction powder with a fiber structure as follows. Figure 2 As shown in (b) and (d) in the middle.

[0073] Step S5: 7 g of Fe / Bi-S@C and 3 g of PVDF (polyvinylidene fluoride) were added to 100 ml of NMP solution (N-methylpyrrolidone) and stirred at room temperature for 2 h to obtain a uniform dispersion. Subsequently, this dispersion was coated onto one side of a commercial membrane by vacuum filtration to deposit Fe / Bi-S@C, and dried at 60 °C for 6 h. Figure 3 As shown in (d) and (e).

[0074] Step S6: The other side of the membrane is coated using the same process. The dispersion is prepared by dissolving 7 g of BN–MXene heterojunction and 3 g of PVDF in NMP, and is used to deposit the BN–MXene heterojunction. Figure 3 As shown in (a) and (b), the resulting Janus composite membrane is as follows. Figure 3 As shown in (c), in the obtained Janus composite membrane, the loading amounts of Fe / Bi-S@C and BN–MXene heterojunctions are 1.5 mg cm⁻¹ on each side. -2 Finally, cut all the coated diaphragms into 17 mm diameter circles or into 6 × 10 cm rectangles.

[0075] Example 2: Assembly of the prepared separators into batteries in a glove box: The Celgard separator and Janus composite separator were assembled with the positive electrode and lithium metal negative electrode of a sulfur battery, and an electrolyte of 1 M LiTFSI, 0.1 M LiNO3, and DOL / DME (v / v=1 / 1) was added. The assembly sequence of the battery was: negative electrode shell—lithium sheet—separator—80 µL electrolyte—positive electrode sheet—stainless steel gasket—spring sheet—positive electrode shell. Finally, the battery was placed in a packaging machine and subjected to a pressure of 5 MPa for 5 s to complete the battery assembly, which was named Celgard lithium-sulfur battery and Janus lithium-sulfur battery, respectively.

[0076] The following performance tests and comparisons of Celgard lithium-sulfur batteries and Janus lithium-sulfur batteries were conducted, specifically including electrochemical tests, lithium dendrite suppression performance tests, and safety performance tests.

[0077] Test Example 1: Interface Adhesion Test To verify the interfacial bonding performance between the coating of the Janus composite membrane and the Celgard base membrane described in this invention, a 180° peel test was used for quantitative testing. The specific steps are as follows: The prepared Janus composite membrane was cut into strips with a width of 10 mm and a length of 100 mm. One end was fixed to a tensile jig with double-sided tape, and the other end was connected to an electronic tensile testing machine. During the test, 180° peeling was performed at a speed of 50 mm / min, and the peel force-displacement curve was recorded. Each sample was tested at least three times, and the average value was taken as the final result. The peel strength of the interface between the Fe / Bi-S@C functional layer and the Celgard base membrane, and the peel strength of the interface between the BN / MXene heterostructure layer and the Celgard base membrane were measured respectively. The test results are as follows: Figure 4 As shown, both the Fe / Bi-S@C layer-Celgard interface and the BN / MXene layer-Celgard interface exhibit high interfacial bonding strength. The average peel strength between the Fe / Bi-S@C layer and the Celgard substrate film is approximately 1.25 N / cm, indicating a stable chemical and mechanical bond between the coating and the substrate film. The peel strength between the BN / MXene layer and the Celgard substrate film is approximately 1.10 N / cm, which is also significantly higher than that of ordinary coated membranes (approximately 0.4 N / cm). Therefore, the composite interface constructed through dopamine surface modification and sol-gel effectively enhances the adhesion between the coating and the substrate film, enabling the Janus composite membrane to maintain good structural integrity and interfacial stability under subsequent electrochemical cycling and thermal stress conditions.

[0078] Test Example 2: Thermal Conductivity Test To demonstrate the effect of the coating layer on the thermal conductivity of the diaphragm, the thermal conductivity of different diaphragms was tested, such as... Figure 5 As shown, the Celgard diaphragm has a thermal conductivity of only 0.28 W / m. -1 K -1 It has poor thermal conductivity. In contrast, the Janus membrane, due to the significantly high thermal conductivity of BN, achieves 51.72 W / m². -1 K -1 The ultra-high thermal conductivity of the battery effectively alleviates the formation of local hot spots and heat accumulation, which not only mitigates the intensified dissolution of LiPSs but also effectively inhibits lithium dendrite growth, thereby improving battery cycle life and safety.

[0079] Test Example 3: Ionic Conductivity Test To measure the ionic conductivity of the Janus composite membrane, a stainless steel / / membrane / / stainless steel battery was assembled, and stainless steel symmetric batteries were assembled using Celgard membrane and Janus composite membrane respectively. The assembled batteries were named Celgard stainless steel symmetric battery and Janus stainless steel symmetric battery respectively.

[0080] The Celgard membrane and the Janus composite membrane show significant differences in ionic conductivity. Test results are as follows: Figure 6 As shown, the ionic conductivity of the Celgard membrane is approximately 0.36 mS / cm. -1 The ionic conductivity of the Janus composite membrane is increased to approximately 0.85 mS / cm. -1 These results demonstrate that introducing Fe / Bi-S@C layers and BN / MXene heterostructure layers on both sides of the Celgard base membrane can effectively improve the wettability of the electrolyte and the interfacial contact state, construct a more continuous ion transport channel, and thus significantly enhance the overall ion conductivity of the membrane.

[0081] Test Example 4: Electrochemical Test like Figure 7 As shown, the cycle performance of the two batteries at 0.1 C was tested. The initial discharge capacity of the Celgard lithium-sulfur battery was 1128.7 mAh·g. -1 After 9 cycles, the concentration dropped to 888.9 mAh·g. -1 After 100 cycles, only 573.9 mAh·g remained. -1 The capacity retention rate was 50.8%. In comparison, the initial discharge capacity of the Janus lithium-sulfur battery was 1350.4 mAh·g. -1 It still maintains 915.4 mAh·g after 100 cycles. -1 The capacity retention rate was 67.8%. The Celgard lithium-sulfur battery's discharge capacities after 30, 50, 80, and 100 cycles were 746.0, 665.5, 606.0, and 573.9 mAh·g, respectively. -1 The Janus lithium-sulfur batteries have capacities of 1128.0, 1043.9, 964.9, and 915.4 mAh·g, respectively. -1 The constant current charge-discharge curves of the two batteries are as follows: Figure 11 As shown in (a) and (b), the capacity (Q) corresponding to the high and low voltage platforms was further analyzed. H and Q L ), the result is as follows Figure 12 As shown in (a) and (b), the Celgard lithium-sulfur battery's Q during the first cycle... H and Q L The values ​​are 305.9 and 822.8 mAh·g, respectively.-1 By the 100th cycle, the concentrations decreased to 187.7 and 386.2 mAh·g. -1 Janus lithium-sulfur batteries' Q H and Q L The concentrations are 307.3 and 1043.1 mAh·g, respectively. -1 Reduced to 247.3 and 668.1 mAh·g -1 Janus lithium-sulfur batteries' Q H With Q L The retention rates were 32.1% and 39.3%, respectively, which were significantly higher than the 18.2% and 24.9% of Celgard lithium-sulfur batteries, indicating that the Janus composite separator can effectively slow down the loss of active materials and promote the Li2S2 / Li2S conversion reaction.

[0082] according to Figure 13 (a) and (b), with Q L / Q H Parameters were used to evaluate the reduction catalytic performance of polysulfides. The Q values ​​of Celgard lithium-sulfur batteries at the 1st, 30th, 50th, 80th, and 100th cycles were compared. L / Q H The values ​​were 2.69, 2.10, 2.00, 1.94, and 2.06, respectively, while the Janus lithium-sulfur battery exhibited a higher Q value. L / Q H Value. Furthermore, with Q... L / 3Q H Characterizing the utilization rate of soluble sulfur, the utilization rates of Celgard lithium-sulfur batteries were 0.90, 0.70, 0.67, 0.65, and 0.69, while those of Janus lithium-sulfur batteries were 1.13, 0.92, 0.88, 0.90, and 0.90, indicating that the Janus composite membrane significantly improved the reversible utilization of sulfur.

[0083] like Figure 8 As shown, cycling tests were conducted under conditions of 0.1–0.5 C. The initial discharge capacity of the Celgard lithium-sulfur battery was 1080.0 mAh·g. -1 When the current density is increased to 0.5 C, it decreases to 591.2 mAh·g. -1 After 150 cycles, it maintained 414.7 mAh·g. -1 The initial capacity of the Janus lithium-sulfur battery is 1291.0 mAh·g. -1 It still maintains 1063.4 mAh·g at 0.5 C. -1 After 150 cycles, the concentration was 814.8 mAh·g. -1 The improvement of approximately 96.5% indicates that the Janus composite membrane significantly improves the kinetics of the sulfur conversion reaction.

[0084] Cyclic performance under 1 C conditions is as follows Figure 9 As shown, the Janus lithium-sulfur battery has an initial capacity of 1232.8 mAh·g at 0.1 C. -1 This is higher than Celgard lithium-sulfur batteries' 937.7 mAh·g. -1 It still maintains 453.9 mAh·g after 500 cycles. -1 Its stable cycling performance and high coulombic efficiency indicate that the modified layer on the diaphragm surface effectively suppresses the polysulfide shuttle effect.

[0085] like Figure 10 As shown, the rate performance of the Janus lithium-sulfur battery was tested at different rates. Its discharge capacities at 0.1, 0.5, 1, 2, and 4C were 1326.9, 1042.9, 827.6, 732.1, and 597.8 mAh·g, respectively. -1 Both figures are higher than those of Celgard lithium-sulfur batteries. When the rate is restored to 0.1 C, the capacity recovers to 1274.5 mAh·g. -1 The retention rate reached 96.1%, indicating that the battery has excellent reversibility.

[0086] Test Example 5: Lithium Dendrite Suppression Performance Test To verify the inhibitory effect of the Janus composite separator on lithium dendrite growth, Li / / separator / / Cu batteries were assembled to observe lithium deposition and stripping behavior and evaluate the separator's inhibitory performance. Lithium-copper batteries were assembled using Celgard and Janus composite separators, respectively, and named Celgard lithium-copper battery and Janus lithium-copper battery, respectively. The results are as follows: Figure 14 As shown in (a).

[0087] The Celgard lithium-copper battery (referred to here as the Celgard lithium-copper battery or Janus lithium-copper battery to distinguish it from the battery in Test Example 1 above) exhibited significant fluctuations in coulombic efficiency during cycling, while the voltage of the Janus lithium-copper battery remained almost stable, suggesting that its coulombic efficiency has higher stability. Specifically, the coulombic efficiencies of the Celgard lithium-copper battery at 25, 50, and 80 cycles were 61.4%, 29.4%, and 37.2%, respectively, while those of the Janus battery were 82.1%, 78.2%, and 80.4%, respectively. These results indicate that the Janus composite separator effectively suppresses dendrite growth and reduces electrolyte and Li⁺ consumption.

[0088] Voltage-time curve as shown Figure 14As shown in (b) and (c), the superiority of the Janus composite separator in suppressing dendrite growth and improving battery safety is further verified. The BN modified layer forms a dense physical barrier, which can uniformly distribute Li⁺ flow, reduce local current density, and suppress dendrite nucleation; the chemical stability of BN material can promote the formation of a dense SEI film and reduce electrolyte decomposition; its good lithiophilicity improves the electrode-electrolyte interface contact and promotes uniform Li⁺ deposition. Combined with the aforementioned test results, the high mechanical strength and thermal conductivity of the Janus composite separator can effectively prevent dendrite penetration and quickly dissipate local heat, thereby preventing the rapid growth of dendrites induced by hot spots. In addition, the highly conductive matrix and the lithiophilic properties of N and S heteroatoms help to reduce lithium transport impedance and achieve uniform distribution of Li⁺ flux.

[0089] Meanwhile, to further evaluate lithium deposition behavior, Li / / membrane / / Li symmetric cells were assembled using Celgard membranes and Janus composite membranes, respectively. The assembled cells were named Celgard lithium symmetric cells and Janus lithium symmetric cells, respectively. The test results are as follows: Figure 15 As shown in (a), (b), and (c), the Celgard lithium-symmetric battery exhibited an overpotential of 244.8 mV after 50 hours of cycling, which then fluctuated to 174.2, 270.8, and 274.8 mV after 68, 85, and 300 hours, respectively, demonstrating significant fluctuations. In contrast, the Janus lithium-symmetric battery showed an overpotential of only 46.5 mV, which remained stable at approximately 50.6 mV after 300 hours without significant change. These results further demonstrate that the Janus composite separator can significantly suppress lithium dendrite growth and improve battery cycle stability.

[0090] Test Example 6: Security Performance Test The tests utilized a thermally accelerated calorimeter (THT-ARC) to test lithium-sulfur batteries with two different separators. During the tests, the THT-ARC operated in a "heat-wait-search" mode, gradually increasing the battery temperature. Heating was stopped when the battery exhibited self-heating and the temperature rise rate exceeded 0.02 °C / min. The activation energy of the battery was calculated using the data obtained from the ARC experiments based on the Arrhenius equation, thus assessing the risk of thermal runaway.

[0091] like Figure 16As shown in (a) and (b), the Celgard lithium-sulfur battery exhibits T0 and T1 values ​​of 169.2 °C and 224.4 °C, respectively, with a total heat release ΔH of 9.3 KJ. In stark contrast, the Janus lithium-sulfur battery demonstrates T0 and T1 values ​​of 243.7 °C and 317.1 °C, representing increases of 74.5 °C and 92.7 °C, respectively, with a total heat release of 12.7 KJ. Furthermore, t0 and t1 are significantly extended by 381.5 and 614.6 min, respectively. These improvements in key thermal stability data further demonstrate the effective enhancement of LSB safety by the modified separator. Figure 17 As shown in (a) and (b), the maximum temperature rise rate of the Janus lithium-sulfur battery is 143.2 °C min, compared to the Celgard lithium-sulfur battery. -1 Reduce to 6.5 ℃ min -1 .

[0092] The activation energies of different battery reactions were further investigated to evaluate the impact of different separators on the thermal safety of LSBs. Lithium-sulfur batteries were assembled using fresh Celgard and Janus composite separators and recycled Celgard and Janus composite separators, respectively, resulting in Celgard (fresh), Janus (fresh), Celgard (cycled), and Janus (cycled) batteries, as shown in Table 1.

[0093] Table 1 Key TR parameters for assembling different separator batteries

[0094] The results are as follows Figure 18 As shown in (a) and (b). Fresh Janus lithium-sulfur batteries E a The value is 0.94 eV, significantly higher than the 0.29 eV of Celgard lithium-sulfur batteries. This demonstrates that under the same abuse conditions, the modified separator makes thermal runaway of LSBs more difficult and reduces the probability of thermal runaway. The key to achieving high-safety batteries is to increase T0, maximize T1, and minimize T2. max Clearly, Janus composite membranes help achieve these goals.

[0095] In patent CN105609690B, a lithium-sulfur battery assembled with a graphene-coated lithium-sulfur battery separator exhibits an initial discharge capacity of 1226 mAh / g at a 0.1C charge / discharge rate, which decreases to 795 mAh / g after 100 cycles. In contrast, the lithium-sulfur battery assembled with the Janus composite separator prepared in this invention achieves an initial discharge capacity as high as 1350.4 mAh / g at the same 0.1C rate, maintaining 915.4 mAh / g after 100 cycles. Comparing the capacity decay before and after cycling reveals that this separator has limited ability to suppress the "shuttle effect" of polysulfides. In contrast, the Janus composite separator designed in this application, with its frog-egg-like hollow structure and nitrogen-sulfur co-doped carbon framework and metal sulfide nanoparticles, achieves efficient adsorption and catalytic conversion of lithium polysulfides, significantly improving the reversible utilization and cycle stability of sulfur species.

[0096] In the literature [C. Chang, C. Yang, Q. Wu, X. Wang, H. Nie, X. Zhou, X. Xie, B.Hwang, Y. Ye, All-in-one Janus separator for lithium–sulfur batteries with lithium polysulfide and dendrite growth suppressed at temperature gradient effect, J. Power Sources 550 (2022) 232115], the disclosed Janus-type LCL-TCL separator has a strength of 0.59 W / m². -1 K -1 The Janus composite membrane of this invention achieves a thermal conductivity of 51.72 W / m² due to the significantly high thermal conductivity of BN. -1 K -1 The ultra-high thermal conductivity of the battery effectively alleviates the formation of local hot spots and heat accumulation, which not only mitigates the intensified dissolution of LiPSs but also effectively inhibits lithium dendrite growth, thereby improving battery cycle life and safety.

[0097] In the literature [B. Yang, R. Pang, J. He, H. Sun, B. Yuan, M. Zhang, Janusseparator with high-temperature resistance and dendrite suppression for advanced Li-ions batteries, J. Power Sources 600 (2024) 234259], the ionic conductivity of the AH@PP Janus composite separator disclosed therein is 0.59 mS / cm. The ionic conductivity of the Janus composite separator of this invention can reach 0.85 mS / cm. This is due to the modified coating on one side composed of specific functional components. Its optimized porous structure and excellent electrolyte-loving properties not only broaden the ion transport channels but also improve the electrolyte wetting efficiency, thereby achieving more efficient ion conduction.

[0098] In patent CN112038552A, the average specific capacity of a bacterial cellulose composite lithium-sulfur battery separator disclosed under different rate cycling at 0.2, 0.5, 1, 3, and 5C is 1150, 946, 746, 445, and 245 mAh / g, respectively. The average specific capacity of the battery assembled with the Janus composite separator of this invention under different rate cycling at 0.1, 0.5, 1, 2, and 4C is 1325, 1040, 820, 730, and 590 mAh / g, respectively. -1 This is thanks to the heterojunction coating on the other side, which is composed of BN-MXene. Its high Young's modulus forms a strong physical barrier that can effectively prevent dendrite penetration.

[0099] In patent CN118978146A, a Co is disclosed. 0.5 Ni 0.5 The Te2 nanoparticle / three-dimensional carbon sheet composite membrane battery maintained a coulombic efficiency of around 60% at 0.5C cycling, while the lithium metal battery using this membrane maintained a stable coulombic efficiency of over 80% during cycling, and the overpotential of the Li / / Li symmetric cell remained stable at approximately 50mV with minimal fluctuation. This indicates that the membrane can uniformly distribute lithium-ion flow, reduce local current density, thereby significantly suppressing lithium dendrite nucleation and growth, reducing irreversible consumption of active lithium and electrolyte, and greatly improving the cycle stability and safety of the battery.

[0100] In patent CN117012954A, a Fe-Sn-S@HC@C separator is disclosed with a melting point of 430°C. The novel separator prepared by this invention has a melting point exceeding 550°C, thus imparting higher thermal stability and safety to the battery.

[0101] In the adiabatic accelerated calorimetry test simulating thermal runaway, the battery equipped with this separator showed a significant increase in the onset thermal runaway temperature (T0) from 169.2℃ for commercial separators to 243.7℃, and the thermal runaway initiation temperature (T1) from 224.4℃ to 317.1℃, with the highest temperature (T... max The temperature rise was increased from 380.3℃ to 532.3℃, while the thermal runaway time was significantly delayed. More importantly, the maximum temperature rise rate decreased sharply from 143.2℃ / min to 6.5℃ / min, and the activation energy of the thermal runaway reaction was as high as 0.94 eV. These key data together demonstrate that the membrane can effectively delay or even prevent the thermal runaway chain reaction.

Claims

1. A method for preparing a multifunctional Janus composite membrane, characterized in that, Includes the following steps: Step S1: Prepare hydrogel microspheres; the raw materials used are chitosan, dopamine, ferric nitrate nonahydrate and bismuth nitrate pentahydrate; Step S2, carbonization and sulfidation treatment; the hydrogel microspheres prepared in step S1 are calcined at high temperature in a nitrogen atmosphere to obtain carbonized aerogel; after calcination, high temperature sulfidation is carried out in a sulfur vapor atmosphere to obtain metal sulfide / carbon composite aerogel with metal sulfide in situ composite on carbon skeleton, denoted as Fe / Bi-S@C; the composite aerogel is centrifuged, washed with anhydrous ethanol and dried. Step S3: Prepare boron nitride nanosheets and MXene nanosheets; Step S4: The MXene nanosheets and boron nitride nanosheets prepared in step S3 are mixed with a bio-based antioxidant and then placed in a vacuum ball mill jar for ball milling. After ball milling, the resulting product is heated and kept warm in an inert atmosphere to obtain BN–MXene heterojunction powder. Step S5: Dissolve the composite aerogel prepared in step S2 and polyvinylidene fluoride in N-methylpyrrolidone solution, stir at room temperature to obtain a uniform dispersion; coat the dispersion onto one side of a commercial membrane by vacuum filtration, deposit metal sulfide / carbon composite aerogel particles on this side of the commercial membrane, and dry it. Step S6: Dissolve the BN–MXene heterojunction powder prepared in step S4 and polyvinylidene fluoride in an N-methylpyrrolidone solution, stir at room temperature to obtain a uniform dispersion; coat the dispersion onto the other side of a commercial membrane by vacuum filtration, deposit the BN–MXene heterojunction on this side of the commercial membrane, and dry it.

2. The method for preparing a multifunctional Janus composite diaphragm according to claim 1, characterized in that, In the Janus composite membrane obtained after step 6, the loading amount of the Fe / Bi-S@C and BN–MXene heterojunctions on both sides of the membrane is 1.5 mg / cm² per side. -2 .

3. The method for preparing a multifunctional Janus composite diaphragm according to claim 1, characterized in that, The specific procedure for step S1 is as follows: 1.5 g chitosan, 0.5 g dopamine, 0.5 g ferric nitrate nonahydrate, and 0.5 g bismuth nitrate pentahydrate are dissolved in 50 mL of 5.0% acetic acid solution, and the solution is magnetically stirred at 180 rpm for 24 h to obtain a homogeneous solution; subsequently, the solution is diluted with 3.5 mL of water per minute. -1 The solution was slowly added dropwise to a 10% sodium hydroxide solution and aged for 24 h. The resulting hydrogel microspheres were washed with deionized water until the filtrate was neutral. Then, the hydrogel microspheres were placed in 100 mL of a 2% glutaraldehyde solution and stirred for 2 h. The product was dried, washed with anhydrous ethanol, and then freeze-dried for 12 h.

4. The method for preparing a multifunctional Janus composite diaphragm according to claim 1, characterized in that, The specific procedure for carbonization in step S2 is as follows: the hydrogel microspheres prepared in step S1 are placed in a tube furnace and heated at 3 °C for 3 min under a nitrogen atmosphere. -1 The temperature was increased to 800 °C and held for 2 h. Then, under continuous nitrogen atmosphere protection, the tube furnace was allowed to cool naturally to room temperature to obtain carbonized aerogel. The specific procedure for vulcanization is as follows: the carbonized aerogel is placed in a tube furnace containing sulfur powder, and nitrogen is continuously introduced. First, the low-temperature zone where the sulfur powder is located is heated to generate sulfur vapor. Then, the high-temperature zone where the carbonized aerogel is located is heated to 600 °C and held for 2 h.

5. The method for preparing a multifunctional Janus composite membrane according to claim 1, characterized in that, The preparation process of boron nitride nanosheets in step S3 includes: Hexagonal boron nitride micro powder was placed in a quartz tube in the center of a tube furnace. The tube furnace was heated from room temperature to 800~1200℃ at a heating rate of 5℃ / min and held at that temperature for 2~4 h. After the tube furnace cooled naturally to room temperature, the quartz tube was removed to obtain the hydroxylated boron nitride product, denoted as BNO. BNO powder was added to a toluene solution, stirred and mixed, and then heated to 90 °C. KH-560 was added, and the mixture was refluxed for 12 h. After the reaction was completed, the reaction solution was cooled to room temperature and then vacuum filtered. The precipitate remaining after filtration was collected and repeatedly washed with ethanol to remove residual toluene solvent and unreacted KH-560, finally obtaining modified boron nitride nanosheets.

6. The method for preparing a multifunctional Janus composite membrane according to claim 1, characterized in that, The preparation process of MXene nanosheets in step S3 includes: HCl solution and LiF powder were added sequentially to a polytetrafluoroethylene (PTFE) beaker to obtain a first solution. Ti3AlC2 powder was then added to the first solution, and the mixture was stirred at 35 °C for 24 h. After the reaction was complete, the resulting mixture was centrifuged at 5000 rpm for 5 min, and the bottom precipitate was collected and washed with deionized water until neutral. The precipitate was then redispersed in Ti3AlC2. x The slurry was ultrasonically stirred, and the upper layer solution was freeze-dried to finally obtain MXene nanosheets.

7. A multifunctional Janus composite membrane, prepared by the method for preparing the multifunctional Janus composite membrane according to claim 1, characterized in that, The diaphragm is composed of different functional layers coated on both sides of the Celgard diaphragm: one side is a Fe / Bi-S@C coating layer, in which hydrogel microspheres are obtained through the sol-gel reaction of chitosan, dopamine and metal salt precursors, and after freeze-drying, carbonization and sulfidation treatment, FeS and Bi2S3 nanoparticles are generated in situ in the carbon framework; the other side is a heterostructure coating layer, which is a stable van der Waals heterostructure layer assembled from BN and MXene nanosheets.

8. A multifunctional Janus composite diaphragm according to claim 7, characterized in that, The loading of Fe / Bi-S@C and BN–MXene heterojunctions on both sides of the Celgard diaphragm was 1.5 mg cm⁻¹ on each side. -2 .

9. Application of a multifunctional Janus composite separator as a battery separator in lithium-ion batteries.

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